Enhancement: Improve `repair packs` command

The `repair packs` command has been improved to also be able to process
truncated pack files. The `check --read-data` command will provide instructions
on using the command if necessary to repair a repository. See the guide at
https://restic.readthedocs.io/en/stable/077_troubleshooting.html for further
instructions.

https://github.com/restic/restic/pull/4644
https://github.com/restic/restic/pull/4655
